WHAT’S UP DOC? Cellulitis

Tuesday

Sep 11, 2018 at 11:45 AMSep 11, 2018 at 11:45 AM

Q: My doctor said the red, warm, tender area that developed on my shin was cellulitis and she prescribed an antibiotic. What is this?

A: Cellulitis is an infection of the skin, and possibly of other soft tissues. It is diagnosed based on the history and especially the physical exam, where redness, warmth, tenderness and swelling of the skin is identified. No other tests are typically needed if the history and physical exam are determined to be sufficient to conclude the cellulitis is uncomplicated (no systemic symptoms and not a purulent cellulitis, see below for more discussion on what this means). However, other conditions that can cause a red, warm, tender, swollen rash (such as Lyme disease and many other conditions) must also be considered.

Another (sometimes associated) condition to consider is development of an abscess. If bacteria is trapped underneath the skin (with or without cellulitis), a collection of pus can develop (from the infection and mostly from the cells trying to fight off the infection) and this is called an abscess (sometimes called a ‘boil’).

A systemic spread of bacteria may be suspected if symptoms such as high fever, low blood pressure, malaise and/or other symptoms, develop. If this occurs in a patient with cellulitis the condition is considered to be more complicated than simply a localized skin infection.

Cellulitis is common, affecting 2 to 25 per 1000 people each year, and is more common in those with risk factors such as a compromised immune system, diabetes, lymphedema, venous insufficiency, obesity and/or other conditions.

Cellulitis is usually divided into purulent cellulitis (with pus-like drainage, and possibly an associated abscess) or non-purulent cellulitis (uncomplicated cellulitis, with no systemic symptoms as discussed below and none of the purulent characteristics).

Cellulitis is thought to be due to the invasion of the skin from a cut or other portal of entry, and is most often due to one of the subtypes of beta-hemolytic streptococcus, although other bacteria can cause it. An abscess is usually due to staphylococcus aureus, although other bacteria can cause this condition as well.

An abscess is usually “drained” is a procedure where an excision is made in the skin to give the infection and the pus a pathway to ‘escape’ from being trapped under the skin (a procedure called an incision and drainage, or I&D, and usually done with a local anesthetic such as lidocaine). If there is no cellulitis, antibiotic therapy after a successful I&D may not be required.

Cellulitis is treated with antibiotics. The specific antibiotic selected depends on the location of the cellulitis, any confounding factors (for example, cellulitis as a complication of an animal bite may raise suspicion of certain bacteria which may guide the antibiotic choice), patient risk factors (such as whether they are diabetic, whether their immune system is functioning normally, as well as other factors) and whether the infection is felt to be localized to the area of skin (the majority of these infections are localized). Uncomplicated, localized cellulitis is typically treated with a course of antibiotics taken orally.

If the cellulitis is suspected to have spread throughout the body (become systemic), blood tests (including a test to look at the number and type of white blood cells, a test to look at the level of inflammation such as a sedimentation rate or C-reactive protein, a blood culture and/or other test, may be indicated). If there is suspicion that the infection has spread to deeper tissues (such as occurs in necrotizing fasciitis, gas gangrene, bone infection or other conditions) then an imaging test (possibly a CT or MRI) may be indicated. If systemic infection is suspected, then treatment with intravenous antibiotics and hospital admission may be appropriate.

With appropriate treatment uncomplicated cellulitis usually begins to respond within a day or two, and if so may be treated with a 5 to 7-day course of oral antibiotics. More complicated infections, or those that respond more slowly (for example because the patient has edema, venous insufficiency or other factors), may require longer treatments.

Unfortunately, cellulitis often recurs (a quarter to half of patients have a recurrence within a year or two), especially in patients with certain risk factors (such as co-infection with certain fungi such as tinea pedis, or patients with lymphedema, venous insufficiency and/or other conditions). Patients with recurrences may be considered for longer courses of antibiotics to ‘suppress’ the recurrence of their infection.